1. Field of the Invention
The invention relates in general to the field of pressure sensing, and in particular to optical pressure sensors for pressure sensing of media, such as gases, liquids, or solids.
2. Related Art
In general, non-optical pressure sensors are known and operate on a principal that is based upon detecting a change in material properties as a function of an applied force. For example, many non-optical pressure sensors use a silicon diaphragm connected to a precision current source and voltage meter. As the applied force changes, the silicon diaphragm changes its resistance in proportion, causing the voltage across the diaphragm to change in direct response. Hence, this voltage allows a direct inference of the applied force, which can be directly related to applied pressure.
A major impediment to non-optical pressure sensors arises from the requirement of electrical current flow at the sensor location. For explosive environments where any form of electrical current could ignite the media, many different schemes of preventing such an event have been developed. Most anti-explosion countermeasures involve changing the environment that contains the media—double insulating the container and placing the sensor in between the two vessels—that often results in tremendous additional cost and weight to the system.
Another significant impediment to non-optical pressure sensors is the close proximity requirement between the sensor diaphragm and the signal processing electronics. In most non-optical systems, the resolution, or the smallest detectable change in pressure, is limited by electrical noise that is induced into the system due to generally harsh operating environments. To overcome this problem with induced noise, system designers have three main options: (1) shorten the distance between the pressure sensor and the signal processing electronics, (2) shield the interconnecting cable between the sensor/electronics, or (3) use a combination of both methods. Typically, (3) is implemented, resulting in increased weight due to the shielding solution as well as added complexity in co-locating the sensor and signal processing functions.
Another area of concern with respect to non-optical pressure sensing methodologies is the adhesive materials used during the manufacturing process. Many temperature and stress/strain-stabilized epoxies use organic binders that release organic compounds as they age. For small-volume pressure measurements this outgassing phenomenon causes the total pressure within the sensor vessel to increase, contributing significantly to the overall error of the system. Additionally, in many instances, these gaseous organic compounds can react with the media they are sensing, causing unwanted chemical reactions, potentially giving rise to self-ignition.
Another major impediment to non-optical pressure sensor methodologies is the long-term stability of such sensors. In general, the majority of non-optical pressure sensing methodologies are termed relative—the measurement is valid for as long as power is applied to the system. If the input power to the system fluctuates significantly, the stability of the measurement system is adversely affected, requiring recalibration of the sensor element. Furthermore, removing power from the system results in complete loss of the calibration reference; this also requires recalibration upon re-energizing the equipment. Both of these scenarios prevent the long-term monitoring of an environment with respect to pressure.